The present invention relates generally to particle impactors and, more particularly, to low particle loss cascade impactors with externally removable collection cups.
Cascade impactors have been used extensively for a number of years to determine the size distribution of aerosol particles. Generally, a cascade impactor has a plurality of collection stages arranged in series, each stage having a nozzle orifice decreasing in size over that of the previous stage and also having an impaction surface for the collection of the particles. The smaller the nozzle orifice, the higher the velocity of air or gas and particles moving through the orifice. The higher the velocity, the smaller the particles that are collected on the impaction plate.
Particles larger than the cut size of the impactor impinge upon the impaction plate. The cut size or cut point of the impactor is the point of 50% collection efficiency. The smaller particles pass with the air stream out of the first impaction region and proceed to the next stage. This procedure continues through the cascade impactor with each stage having higher velocity and collecting smaller size particles. Therefore, the particles collected on the various stages of the impactor will be larger than the cut size of that particular stage and less than the cut size of the stage preceding it. Often, cascade impactors will collect particles upon a flat plate and these particles are analyzed by weighing them and the plate and subtracting the tare weight of the plate. Alternatively, the amount of material deposited on the plate is determined by quantitative chemical analysis. With this latter technique, it would be desirable to collect particles directly into a container where the particles can be analyzed.
A second problem is that particles may be collected on surfaces other than the impaction plates. This is known as interstage particle loss. The reasons for interstage losses is that the airflow passing from one stage to the next must make a series of bends due to the shape of the impaction area and the shapes of the passages leading from one stage to the next stage. For example, in the conventional cascade impactor, the air impinges upon an impaction plate, traverses tangentially outward, and then proceeds downward into the next stage where the flow is directed toward the center and then downward through the nozzles in the next stage of the impactor. The result of these turns is the increased opportunity for the particles to be lost either by impaction or turbulent deposition.